1. Field of the Invention
This invention relates to the determination of CO.sub.2, O.sub.2 and Cl in body fluids such as blood, and more particularly to an improved method and apparatus for performing such measurements reliably, cheaply, and quickly.
2. Description of the Prior Art
The determination of blood gases is important for clinical analysis. In particular, the determination of carbon dioxide (CO.sub.2) and oxygen (O.sub.2) content in whole blood and blood serum are among the most frequently performed analyses in a clinical laboratory. Due to the great importance of these analyses, a number of techniques have been developed and are presently being used to determine CO.sub.2 and O.sub.2 concentration.
The traditional technique used for the determination of CO.sub.2 in blood is the method of D. D. Van Slyke which was published in the Journal of Biological Chemistry, Vol. 61, page 523 (1924). In the basic Van Slyke method, blood serum and acid are mixed in a closed volume and the carbon dioxide in the blood is extracted from the blood by application of vacuum. The extracted carbon dioxide is then measured volumetrically or manometrically. When the vacuum is drawn, other blood gases are released from the serum in addition to the carbon dioxide. This requires that a base, such as sodium hydroxide, be added in order to separate the carbon dioxide from the other released gases. After this, the volumetric measurement is performed by known techniques.
The Van Slyke method is practiced in numerous devices, as for instance in the microgasometers described by S. Natelson in U.S. Pat. Nos. 2,680,060 and 3,171,722. These microgasometers are described in more detail in publication P-5/64, distributed by Scientific Industries, Inc., 220-05 97th Avenue, Queens Village 29, New York, N.Y.
Natelson describes various techniques for connecting his apparatus to a gas chromatograph so that gases liberated from the blood serum may be analyzed in the gas chromatograph. Of course, considerable numbers of papers have been presented which describe the analysis of blood serum for carbon dioxide and other gases using gas chromatography. One of these articles is L. E. Farhi et al. "Determination of Dissolved N.sub.2 in Blood by Gas Chromatography and (a-A) N.sub.2 Difference", Journal of Applied Physiology, Vol. 18, No. 1, pages 97-106, January 1963. Another article of interest is T. Johns and B. Thompson "Gas Chromatographic Determination of Blood Gases", The Analyzer, Vol. 4, No. 2, April, 1963.
In gas chromatography, the released gases are typically carried in a gas stream over a chromatographic column and then through a detector. The gas stream used as the carrier is usually He, or some other gas having a different thermal conductivity than the gases which are to be measured. The column has different retention rates for different gases in the gas stream and acts to separate the different gases from one another. The detector is a flow detector which provides different peak responses for different gases to be measured. The detector is usually comprised of a hot filament wire whose resistance changes in accordance with the thermal conductivity of the gas which is in contact with the wire. Since the thermal conductivities of the gases to be measured are different from one another, and since the gases have been separated in a known order, each of the transient peaks of the detector response can be associated with one of the gases to be measured. These transient responses are usually plotted on a recorder, since the measurement is a dynamic one done in accordance with the flow of gases, rather than a stationary gas measurement. The integral under the response curve or the peak height of the response curve is then a measure of the gas concentration in the blood sample.
While there have been numerous publications on gas chromatography for determination of, for instance, CO.sub.2 in blood serum, this technique has not found wide application in routine laboratory measurements. The technique is complex, requiring a significant amount of apparatus including the chromatograph and column, together with recording equipment. Additionally, the method is very time consuming. Part of the time consumption is due to the burdens placed on the operator of the apparatus, who has to inject the blood sample, and then wait until the sample passes through the column and the detector. The operator then has to relate the recorder output to the signal from a calibration sample, all of which is time consuming and which can lead to human error.
In addition to the disadvantages noted above, gas chromatography requires the use of a carrier gas stream. This is a dynamic measurement rather than a static measurement, and is consequently more complex and thus less reliable. With such a dynamic process, constant flow rates are required and transient responses have to be quickly recorded in order to provide accurate results.
In addition to those disadvantages, the carrier gas generally has to be a gas having a different thermal conductivity than the gas species to be detected, in order that the measurement of the detected gas species is not altered by the presence of the carrier gas. It is for this reason that gases such as He, which has a significantly different thermal conductivity than air, O.sub.2, N.sub.2, etc are used.
Another disadvantage of most prior art techniques for measuring blood gases concerns the use of a vacuum when the reagent and blood react to release the gases to be detected. Use of a vacuum means that species other than the gas to be measured (such as CO.sub.2) will be released. For instance, O.sub.2, N.sub.2, etc. will be released from the blood and will contaminate the sample measurement where it is desired to measure CO.sub.2.
Still another disadvantage of the use of vacuum relates to the possibility of leakage and lack of vacuum tightness. In vacuum systems, errors generally occur because apparatus such as valves and stopcocks develop leaks. Since the vacuum apparatus is designed to operate reliably only when reproducibly good vacuum is provided, such techniques are critically dependent on the reliability of components which are themselves subject to numerous problems. Consequently, it is important to provide a technique which suffers only minimal interference from dissolved gases in the blood other than the species which is to be measured.
Reference is also made to an instrument for measurement of CO.sub.2, which was evaluated and reported on in Clinical Chemistry, Vol. 19, No. 10, 1227 (1973). This instrument is the "Harleco CO.sub.2 Apparatus", sold by Harleco Co., Philadelphia, Pa. 19143. In this instrument, the gas is released into air and is measured by a volume displacement. The released gas is not pushed to a detector by a reagent, as in the present invention, and the instrument itself is very difficult to clean after each measurement. Also, at least 1.0 ml of serum or plasma is required, and therefore the instrument is not usable for most pediatric samples. Due to friction, the weight of the piston etc., the instrument has to be calibrated frequently.
In general, the prior art methods for measuring blood gases contain certain "non-equilibrium" features which lead to errors. For instance, the application of vacuum extracts gas from the blood which is partly reabsorbed by the blood when the vacuum is removed.
Also, the gas chromatographic methods involve the flow of gas through the system containing blood serum and gas. This causes a change in equilibrium of the system as the sample gas is swept away. That is, no time is available for the flowing gas to establish equilibrium between the flowing gas and the blood sample or at best only a partial equilibrium is obtained, so that it is difficult to establish a basis for the system in order to make a proper measurement.
In contrast with the prior art, the present invention uses an equilibrated, stationary system to obtain more accurate results, where very small samples can be evaluated. The apparatus is automatically cleaned after each measurement, thus insuring good accuracy and rapid measurement. Further, the blood gas measured by the detector has essentially the same composition as that originally established in the vessel, thereby insuring increased accuracy.
Accordingly, it is a primary object of this invention to provide a technique for low cost, reliable measurement of CO.sub.2 and O.sub.2 in body fluid samples such as whole blood and blood serum.
It is another object of this invention to provide an apparatus for measuring the CO.sub.2 and O.sub.2 content of body fluids under stationary equilibrium conditions.
It is still another object of this invention to provide an apparatus for the measurement of CO.sub.2 and O.sub.2 in body fluids which apparatus can be easily flushed after each measurement to provide increased reliability.
It is a further object of the present invention to provide a technique for the determination of CO.sub.2 and O.sub.2 in body fluids which does not require the need for measurement of transient responses nor the recording of such transients.
It is a still further object of this invention to provide a method for measuring CO.sub.2 and O.sub.2 in body fluids such as blood which does not require large sample volumes.
It is still another object of this invention to provide a method for measurement of CO.sub.2, O.sub.2 and Cl in body fluids without requiring volumetric or manometric measurement.
It is another object of this invention to provide a technique for measurement of CO.sub.2 and O.sub.2 in body fuids which provides such measurements very quickly.